Disintegration Mechanism and Hydrogeochemical Processes of Red-Bed Soft Rock Under Drying-Wetting Cycle

: Red-bed soft rock in the drawdown area on bank slopes of landslide easily 23 disintegrates upon exposure to water, and its properties experience comprehensive 24 deterioration, which will cause bank slope instability. To better study disintegration 25 mechanism of the red-bed soft rock, a series of laboratory tests were conducted in this 26 paper to investigate the disintegration characteristics, durability and hydrogeochemical 27 process of red-bed argillaceous siltstone under drying-wetting cyclic conditions. 28 Experimental results showed that, with increasing number of drying-wetting cycles, 29 red-bed argillaceous siltstone gradually disintegrated, from initial appearing the cracks 30 on the surface of the samples to large particles gradually breaking up into small 31 fragments. Significant changes in grain size distribution, and the durability index of the 32 samples progressively decreased. Microstructural analysis showed that the size and 33 distribution of pores and cracks in the sample surface significantly increased, such that 34 the sample surface became disordered and complicated. Notable changed in 35 concentrations of ions in the soaking solutions indicated continuous mineral dissolution 36 and loss during the cyclic drying-wetting. Based on the results obtained from the 37 experiment, it is concluded that the disintegration of samples undergoing drying- 38 wetting cycles was the result of the synergistic action of water and temperature. To be 39 specific, the dissolution of calcite, albite, gypsum, montmorillonite and kaolinite during 40 the wetting procedure, which promotes the decrease in mineral content and increases in 41 pores and cracks. The increases in temperature and the dehydration shrinkage of sample 42 during the drying procedure accelerated the disintegration of the samples.

Abstract: Red-bed soft rock in the drawdown area on bank slopes of landslide easily 23 disintegrates upon exposure to water, and its properties experience comprehensive 24 deterioration, which will cause bank slope instability. To better study disintegration 25 mechanism of the red-bed soft rock, a series of laboratory tests were conducted in this 26 paper to investigate the disintegration characteristics, durability and hydrogeochemical 27 process of red-bed argillaceous siltstone under drying-wetting cyclic conditions. 28 Experimental results showed that, with increasing number of drying-wetting cycles, 29 red-bed argillaceous siltstone gradually disintegrated, from initial appearing the cracks

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Disintegration is a physical and chemical weathering process that is affected by 59 many factors. Phienwej (1995) found that disintegration was caused mainly by changes 60 in the water content of rock. Similar results were found by Newman (1983) and Liu et 61 al. (2000), the results showed that disintegration of mudstone was caused by the 62 presence of water, which reduced the degree of cementation among mineral particles. 63 Youn and Tonon (2010) considered that the disintegration is the structural degradation  drying process is considered to be one of the main processes that can cause degradation 93 and deterioration of rock material.

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As for the red-bed soft rock in the drawdown area on bank slopes of landslide, the 95 cyclic drying-wetting process lasts for a long time during the course of reservoir 96 operation and has its own distinct drying-wetting alternating regime (Zhang et al. 2018).

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Additionally, the disintegration of the red-bed soft rock in the drawdown area is a 98 typical physical and chemical process under cyclic drying-wetting conditions. Although   The disintegration experiment, as described by the ASTM and the ISRM, was 128 performed to assess the disintegration behavior of red-bed argillaceous siltstone in the 129 laboratory. In this study, ten cycles were designed and performed to ensure that all the In each cycle, the samples were submerged in deionized water for 24h to wet state and 133 then, removed and dried in an oven at 105℃ for 24 h according to ASTM D4373-14 134 (2014).

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In this study, 20 samples, were selected for the experiments. A total of 10 drying-136 wetting cycles were carried out for each sample, and the disintegration characteristics

Aqueous solution detection
Different chemical processes can occur during the drying-wetting cycles, which 155 include dissolution/precipitation, ion exchange processes, oxidation, and reduction. 156 Minerals present in rock will partially or completely dissolve in water according to its 157 resistance to chemical weathering. During these chemical interaction processes, the   188 The evolution of the grain content with various particle sizes as the drying-wetting 189 cycles increased is presented in Fig. 4. 190 In Fig. 4, it can be observed that the evolutions of particle content of the various 191 particle sizes under the drying-wetting cycles share an inconsistent trend. To be specific, 192 an insignificant variation in the content of particles that are larger than 5 mm could be 193 detected in the initial stage of the experiment, followed by a rapid decrease as the 194 experiment progressed. However, the rate of decrease in the later period gradually 195 slowed down and stabilized, and the content of this particle size was approximately 2%. 196 The evolution trend of the content of particles that are smaller than 0.25 mm is contrary 197 to that of particles that are larger than 5 mm. It remained almost unchanged during the first three drying-wetting cycles of the samples, which indicates the limited influence 199 of water on the samples at this stage. As the drying-wetting cycles increased to 10, the water-stability of the red-bed argillaceous siltstone increased when its particle size was 210 reduced to less than 2 mm. (1)

Particle analysis of disintegration products
In Eq. (1), Idn is the durability index after the nth cycle, m0 is the initial dry weight 221 of the sample, (g), and mn is the weight of the retained portion of the samples after the 222 nth cycle, (g).

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The values of the durability index Id1-Id10 obtained from different cycles, are

Microstructure characteristics
To monitor the evolution of the microstructure in the tested samples after exposing 243 them to the drying-wetting cycles, SEM at 1000× magnification was used on samples 244 after 0, 1, 3, 5, 7 and 10 cycles. The results are presented in Fig. 6. 245 Fig. 6(a) shows the initial microstructure characteristics of the tested sample.

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Before experiencing drying-wetting cycles, the sample had a dense structure, with few smoother under the water-rock interaction.

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As mentioned above, structures in the red-bed argillaceous siltstone, including 263 pores and cracks, notable changed under cyclic drying-wetting conditions. It can be observed in Fig. 6(a) that, the initial microstructure of the red-bed argillaceous siltstone 265 had a dense structure, with few flaky aggregates and pores distributed on the surface.

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With the increasing number of drying-wetting cycles, the size and distribution of pores

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The results in Figs. 7 and 8  As mentioned previously, the intact samples gradually disintegrated, and the 321 content of disintegration products with different particle sizes drastically changed, 322 during the experimental process. With the increasing number of drying-wetting cycles, 323 the size and distribution of cracks and pores in the microstructure of sample gradually 324 increased, the surface of sample was no longer dense and uniform. Furthermore, 325 according to the results obtained by XRD, presented in Fig. 1 and Table 2, the red-bed 326 argillaceous siltstone analyzed in this study contains many soluble minerals and clay 327 minerals, mainly include calcite, albite, gypsum, montmorillonite and kaolinite, which 328 easily reacts with deionized water, then dissolves and disperses into the aqueous 329 solution, as schematically shown in Fig. 9. Meantime, the expansion force generated     Table 1. Basic physical properties of tested samples. 571 Table 2. Mineral components of tested samples.    Grain content evolutions of various particle sizes.

Figure 5
Evolutions of durability Index (Id) with number of drying-wetting cycles.    Schematic diagram of chemical water-rock interaction.